BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 depicts a safety switch according to an embodiment of the present invention;
FIGS. 2
a and 2b depict a cam arrangement of the safety switch of FIG. 1;
FIG. 3 depicts operating principles of the safety switch of FIG. 1;
FIG. 4 depicts a plunger according to an embodiment of the present invention; and
FIGS. 5, 6 and 7 depict further operating principles of embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 depicts a plan view of a safety switch according to an embodiment of the present invention. The safety switch comprises a two-part housing. One part of the housing defines a main body I of the safety switch. Mounted within the body I are electrical contacts which are fixed in position relative to the body 1. The contacts consist of two fixed safety contacts 2 and a fixed auxiliary contact 3. Also mounted within the body 1 is a plunger 4 which is slideable relative to the body 1 in an axial direction. The plunger 4 is provided with a plurality of contacts which are moveable relative to the plunger 4. The moveable contacts comprise two moveable safety contacts 2a and a moveable auxiliary contact 3a. By moving the plunger 4, the moveable contacts 3a, 4a can be brought into contact (and thus electrical connection) with the fixed contacts 3, 4 of the safety switch. The plunger 4 is also provided with a moveable insulating barrier 11 which serves to provide additional electrical insulation for some of the moveable safety contacts 2a.
The plunger 4 is biased by a spring 5 towards a second part of the housing, which forms a head 6 of the safety switch. The head 6 of the safety switch is provided with a rotatable cam arrangement 7. The cam arrangement 7 is arranged to receive and engage with an actuator (not shown in FIG. 1). Engagement or disengagement of the actuator with the cam arrangement 7 causes the cam arrangement 7 to rotate, which in turn causes axial movement of the plunger 4 within the body 1 of the safety switch.
FIGS. 2
a and 2b illustrate the interaction between the cam arrangement 7 and the plunger 4 in more detail. FIG. 2a shows that the cam arrangement 7 defines a cam surface 8. The cam surface 8 is provided with an indentation 8a which is (upon rotation of the cam arrangement 7) arranged to receive the plunger 4. The cam arrangement 7 is also provided with a notch 9 for receiving and engaging with an actuator. It can be seen from FIG. 2a that when no actuator is brought into engagement with the cam arrangement 7, the cam arrangement pushes back against the plunger 4 (which is biased toward the cam arrangement 7 by the spring 5) and prevents the plunger 4 from moving towards the cam arrangement 7. The plunger 4 is said to be in a first plunger position.
It can be seen from FIG. 1 (in combination with FIG. 2a) that when no actuator is brought into engagement with the cam arrangement 7 all of the fixed safety contacts 2 of the body 1 of the safety switch are kept apart from all of the moveable safety contacts 2a of the plunger 4. Thus, when no actuator is engaged with the cam arrangement 7, the safety contacts 2, 2a are not in electrical connection with each other, which prevents the safety switch from conducting electricity (to, for example, electrically powered machinery with a machine guard). When no actuator is engaged, the auxiliary contacts 3, 3a are in contact with each other, which may allow an auxiliary power supply to be supplied to the switch (for example, to power a light which indicates that no actuator has been engaged with the switch).
FIG. 2
b depicts an actuator 10 that has been brought into engagement with the cam arrangement 7. It can be seen from FIG. 2b that when the actuator 10 has been brought into engagement with the cam arrangement 7, the cam arrangement 7 and therefore cam surface 8 is arranged to rotate in a clockwise direction. Rotation of the cam arrangement 7 causes the indentation 8a in the cam surface 8 to be brought into alignment with the plunger 4. As the indentation 8a moves into alignment with the plunger 4, which is biased by the spring 5, the plunger 4 moves towards the right of FIG. 2b. The plunger 4 is said to be in a second plunger position.
FIG. 3 shows the safety switch with an end cap 6a enclosing the head 6 of the safety switch. The end cap 6a protects the cam arrangement 7 from damage, dust etc, and makes the safety switch appear more aesthetically pleasing. It can be seen from FIG. 3 that when the actuator 10 is brought into engagement with the cam arrangement 7, the plunger 4 moves towards the right of FIG. 3. When the plunger 4 moves to the right, all of the moveable safety contacts 2a are brought into electrical connection with the fixed safety contacts 2 of the body 1 of the safety switch. When all of the safety contacts 2, 2a are brought into electrical connection with each other, the switch is capable of conducting electricity (to, for example, electrically powered machinery with a machine guard). The safety switch is configured such that if one or more of the safety contacts 2, 2a are not in electrical connection with each other, the switch is incapable of conducting electricity. The significance of this configuration will be described further below.
It can be seen from FIGS. 1, 2 and 3 that movement of the plunger 4 towards the right of the Figures allows the safety switch to conduct electricity, since the safety contacts 2, 2a are brought into electrical connection with each other. In the prior art safety switches, if the cam arrangement were to fail (e.g. become worn, cracked, displaced, removed etc.) then the plunger, which is biased by the spring, will continue to be biased towards the right of the Figures. Thus, in prior art safety switches, if the cam arrangement fails, the safety contacts will be brought into connection with one another, and maintained in that position. Specifically, if the cam arrangement fails, the prior art safety switch will fail to danger, in that the failure of the cam arrangement will leave the switch in a configuration in which it is still capable of conducting electricity. This problem is overcome by the present invention due to the design of the plunger 4, as shown in more detail in FIG. 4.
FIG. 4 illustrates a plunger 4 according to an embodiment of the present invention. As already described above, the plunger 4 is provided with moveable safety contacts 2a, and a moveable auxiliary contact 3a. The plunger is also provided with a moveable insulating barrier 11 which serves to provide additional electrical insulation for some of the moveable safety contacts 2a.
The moveable contacts 2a, 3a and insulating layer 11 extend through the plunger 4 in a direction perpendicular to the length of the plunger 4. The moveable contacts 2a, 3a and insulating barrier 11 are moveable along parts of the length of the plunger 4. Movement of the moveable contacts 2a, 3a and insulation barrier 11 is restricted by windows 12, 13 provided in the plunger 4 (more clearly visible in FIGS. 5 and 6), and through which the moveable contacts 2a, 3a and insulation barrier 11 extend. The moveable contacts 2a, 3a and insulation barrier 11 are separated from contacting each other within each window 12, 13 by way of helical springs 14, 15 (more clearly visible in FIGS. 5 and 6). The helical springs 14, 15 ensure that one of the moveable safety contacts 2a and insulation barrier 11 of one window 12 are biased apart from each other, and that one of the moveable safety contacts 2a and moveable auxiliary contacts 3a of the other window 13 are also biased apart from each other. The helical springs 14, 15 bias the moveable safety contacts 2a, auxiliary contact 3a and insulation barrier 11 to the ends of their respective windows 12, 13. The features of the plunger 4 described so far are well known in the art.
The plunger 4 according to an embodiment of the present invention differs from prior art plungers in that it is provided with two frames 16, 17 that extend perpendicularly away from the surface of the plunger 4, and which are fixed in position relative to the plunger 4. The frames 16, 17 may be formed integrally with the plunger 4. The frames are substantially rectangular in shape, in that they define three sides of a rectangle, the fourth and final side of the rectangle being formed by the plunger 4 itself. The moveable safety contacts 2a (or structures to which the moveable safety contacts 2a are attached, e.g. an insulating layer) are each provided with a lip 18, 19 which extends in a direction substantially perpendicularly away from the plunger 4.
The plunger 4 and its constituent parts may be formed from any suitable material. For example, the plunger 4 and its constituent parts may be formed from plastic.
When the moveable safety contacts 2a are moved relative to the plunger 4, they are also moved relative to the frames 16, 17 because the frames 16, 17 are fixed in position relative to the plunger 4. When the moveable safety contacts 2a have been moved by a predetermined amount (which is set to correspond to a failure of the cam arrangement), the lips 18, 19 of the moveable safety contacts 2a press against the outermost extent of the frames 16, 17. When the lips 18, 19 of the moveable safety contacts 2a come into contact with the frames 16, 17, the movement of the moveable safety contacts 2a can be affected. Specifically, movement of the moveable safety contacts 2a can be affected in such a way as to overcome the problems of the prior art discussed above. The function of the plunger 4 according to an embodiment of the present invention is highlighted in FIG. 5.
FIG. 5 illustrates the safety switch according to an embodiment of the present invention. In this particular example, the cam arrangement has failed in that the cam arrangement has been displaced due to the head 6 of the safety switch becoming detached from the body 1. FIG. 6 shows the safety switch of FIG. 5 in cross section so that the operation of the plunger 4 and the movement of the moveable safety contacts 2a may be more easily seen.
Referring back to FIG. 2b, it was described how the plunger 4 moved from a first position when no actuator 10 was engaged with the cam arrangement 7, to a second position when an actuator 10 was brought into engagement with the cam arrangement 7. When the cam arrangement 7 fails (e.g. when there is no cam arrangement at all) the plunger 4 is biased by the spring 5 into a third position which is beyond the position to which the plunger has moved in FIG. 2b—i.e. when the cam arrangement 7 fails the cam arrangement 7 is no longer restricting the movement of the plunger 4 away from the spring 5. In prior art safety switches, this would result in the moveable safety contacts 2a of the plunger 4 staying in electrical connection with the fixed safety contacts 2 of the body 1 of the safety switch. However, due to the frames 16, 17 and lips 18, 19 of the plunger of the present invention, this situation is avoided.
It can be seen from FIGS. 5 and 6 that when the cam arrangement fails, the plunger 4 extends even further towards the end of the switch where the head 6 was originally located. Such movement of the plunger 4 causes the frames 16, 17 of the plunger to be brought into physical contact with the lips 18, 19 of the moveable safety contacts 2a. When the frames 16, 17 of the plunger 4 are brought into contact with the lips 18, 19 of the moveable safety contacts 2a, the moveable safety contacts 2a are moved by the frames 16, 17.
It can be seen from FIGS. 5 and 6 that the moveable safety contacts 2a are moved by the frames 16, 17 such that not all of the moveable safety contacts 2a of the plunger 4 are in contact with the fixed safety contacts 2 of the body 1 of the safety switch. Specifically, the lips 18, 19 and frames 16, 17 cause the moveable safety contacts 2a to be pivoted about one of the fixed contacts 2 of the body 1 of the safety switch, such that one end of the moveable safety contacts 2a is rotated relative to the other. This is shown more clearly in FIG. 7. Since not all of the safety contacts 2, 2a are in electrical connection with each other, the safety switch will not conduct electricity. Thus, by using a plunger 4 according to an embodiment of the present invention, the safety switch no longer fails to danger when the cam arrangement fails, but instead fails to a safe (i.e. non-conducting) configuration.
It will be appreciated that the plunger of the present invention may be used in existing safety switches. For example, the plunger may be retrofitted to safety switches which have already been installed, replacing a prior art plunger. A prior art safety switch can therefore be made to fail-safe by replacing the prior art plunger with the plunger of the present invention. Alternatively, a safety switch incorporating a plunger according to the present invention may be manufactured, sold etc. as a single unit.
In the embodiments described above, a plurality of safety contacts has been described. However, it will be appreciated that any suitable configuration of safety contacts (and even auxiliary contacts) may be employed. For example, a plunger may be provided with only a single safety contact, and not two as shown in the Figures.
It will be understood by the skilled person that a contact is a conductor which may be shaped at each of its ends, i.e. to define contact points. In the above-described embodiments, the moveable safety and auxiliary contacts are conductors which extend transversely through the plunger, and protrude from both sides of the plunger. The fixed contacts are conductors fixed in position relative to the housing of the safety switch.
In the above-described embodiments, a frame 16 has been described as the structure responsible for affecting the movement of the moveable safety contacts 2a by incorporation of a lip 18 on or adjacent to the moveable safety contacts 2a. However, it will be appreciated that the use of a frame fixed in position on the plunger is not necessary, nor is the use of a lip on the moveable safety contacts. Preferably, the plunger is provided with a structure that is fixed in position on the plunger and the structure is positioned such that it is arranged to come into physical contact with and effect the movement of the moveable conductor (i.e. contacts) of the plunger when there is a predetermined amount of relative movement between the plunger and the moveable conductor. The predetermined amount of movement can be selected to correspond to a failure of the cam arrangement, so that at least a part of the moveable conductor (i.e. moveable safety contact) is brought out of electrical connection with the fixed contacts of the safety switch when the cam arrangement fails. The structure can be any suitable structure, for example an arm or protrusion extending from the plunger. The moveable conductor could be provided with any suitable abutment surface or latching surface arranged to come into physical contact with the structure. For example, the moveable conductor could be provided with an arm that extends from the plunger, the arm being provided with a hook at an end of the arm remote from the plunger.
The plunger of the present invention has been described in relation to a safety switch having a fixed set of contacts located and fixed in position in the housing of the safety switch. The fixed contacts of the housing may be individually fixed or integral to the housing, or may form part of a safety switch contact block. The safety switch contact block is a structure that is provided with the fixed contacts (or conductors). The safety switch contact block is generally fixed in position into the housing. Accordingly, it is envisioned that the fixed safety contacts (conductors) may be formed integrally with the housing, individually fixed in position in the housing, or form part of a contact block which is itself fixed in position in the housing.
It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, and that various modifications may be made to those and other embodiments without departing from the invention, which is defined by the claims which follow.
Patent Application
20080023307
